Rewrite tests for declare_interior_mutable_const and

`borrow_interior_mutable_const` to be more thorough on weird constructs

Author: Jarcho

tests/ui/borrow_interior_mutable_const/auxiliary/helper.rs renamed to tests/ui/auxiliary/interior_mutable_const.rs

@@ -0,0 +1,221 @@
+//@aux-build:interior_mutable_const.rs
+
+#![deny(clippy::borrow_interior_mutable_const)]
+#![allow(
+    clippy::declare_interior_mutable_const,
+    clippy::out_of_bounds_indexing,
+    const_item_mutation,
+    unconditional_panic
+)]
+
+use core::cell::{Cell, UnsafeCell};
+use core::ops::{Deref, Index};
+
+trait ConstDefault {
+    const DEFAULT: Self;
+}
+impl ConstDefault for u32 {
+    const DEFAULT: Self = 0;
+}
+impl<T: ConstDefault> ConstDefault for Cell<T> {
+    const DEFAULT: Self = Cell::new(T::DEFAULT);
+}
+
+fn main() {
+    {
+        const C: String = String::new();
+        let _ = C;
+        let _ = &C;
+        let _ = C.len();
+        let _ = &*C;
+    }
+    {
+        const C: UnsafeCell<u32> = UnsafeCell::new(0);
+        let _ = C;
+        let _ = &C; //~ borrow_interior_mutable_const
+        let _ = C.into_inner();
+        let _ = C.get(); //~ borrow_interior_mutable_const
+    }
+    {
+        const C: Cell<u32> = Cell::new(0);
+        let _ = C;
+        let _ = &C; //~ borrow_interior_mutable_const
+        let _ = &mut C; //~ borrow_interior_mutable_const
+        let _ = C.into_inner();
+
+        let local = C;
+        C.swap(&local) //~ borrow_interior_mutable_const
+    }
+    {
+        const C: [(Cell<u32>,); 1] = [(Cell::new(0),)];
+        let _ = C;
+        let _ = &C; //~ borrow_interior_mutable_const
+        let _ = &C[0]; //~ borrow_interior_mutable_const
+        let _ = &C[0].0; //~ borrow_interior_mutable_const
+        C[0].0.set(1); //~ borrow_interior_mutable_const
+    }
+    {
+        struct S(Cell<u32>);
+        impl S {
+            const C: Self = Self(Cell::new(0));
+        }
+        impl Deref for S {
+            type Target = Cell<u32>;
+            fn deref(&self) -> &Self::Target {
+                &self.0
+            }
+        }
+        let _ = S::C;
+        let _ = S::C.0;
+        let _ = &S::C; //~ borrow_interior_mutable_const
+        let _ = &S::C.0; //~ borrow_interior_mutable_const
+        S::C.set(1); //~ borrow_interior_mutable_const
+        let _ = &*S::C; //~ borrow_interior_mutable_const
+        (*S::C).set(1); //~ borrow_interior_mutable_const
+    }
+    {
+        enum E {
+            Cell(Cell<u32>),
+            Other,
+        }
+        const CELL: E = E::Cell(Cell::new(0));
+        const OTHER: E = E::Other;
+
+        let _ = CELL;
+        let _ = &CELL; //~ borrow_interior_mutable_const
+        let E::Cell(_) = CELL else {
+            return;
+        };
+
+        let _ = OTHER;
+        let _ = &OTHER;
+        let E::Cell(ref _x) = OTHER else {
+            return;
+        };
+    }
+    {
+        struct S<T> {
+            cell: (Cell<T>, u32),
+            other: Option<T>,
+        }
+        impl<T: ConstDefault + Copy> S<T> {
+            const C: Self = Self {
+                cell: (Cell::<T>::DEFAULT, 0),
+                other: Some(T::DEFAULT),
+            };
+
+            fn f() {
+                let _ = Self::C;
+                let _ = &Self::C; //~ borrow_interior_mutable_const
+                let _ = Self::C.other;
+                let _ = &Self::C.other;
+                let _ = &Self::C.cell; //~ borrow_interior_mutable_const
+                let _ = &Self::C.cell.0; //~ borrow_interior_mutable_const
+                Self::C.cell.0.set(T::DEFAULT); //~ borrow_interior_mutable_const
+                let _ = &Self::C.cell.1;
+            }
+        }
+    }
+    {
+        trait T {
+            const VALUE: Option<Cell<u32>> = Some(Cell::new(0));
+        }
+        impl T for u32 {}
+        impl T for i32 {
+            const VALUE: Option<Cell<u32>> = None;
+        }
+
+        let _ = &u32::VALUE; //~ borrow_interior_mutable_const
+        let _ = &i32::VALUE;
+    }
+    {
+        trait Trait<T: ConstDefault> {
+            type T<U: ConstDefault>: ConstDefault;
+            const VALUE: Option<Self::T<T>> = Some(Self::T::<T>::DEFAULT);
+        }
+        impl<T: ConstDefault> Trait<T> for u32 {
+            type T<U: ConstDefault> = Cell<U>;
+        }
+        impl<T: ConstDefault> Trait<T> for i32 {
+            type T<U: ConstDefault> = Cell<U>;
+            const VALUE: Option<Cell<T>> = None;
+        }
+
+        fn f<T: ConstDefault>() {
+            let _ = &<u32 as Trait<T>>::VALUE; //~ borrow_interior_mutable_const
+            let _ = &<i32 as Trait<T>>::VALUE;
+        }
+    }
+    {
+        trait Trait {
+            const UNFROZEN: Option<Cell<u32>> = Some(Cell::new(0));
+            const FROZEN: Option<Cell<u32>> = None;
+            const NON_FREEZE: u32 = 0;
+        }
+        fn f<T: Trait>() {
+            // None of these are guaranteed to be frozen, so don't lint.
+            let _ = &T::UNFROZEN;
+            let _ = &T::FROZEN;
+            let _ = &T::NON_FREEZE;
+        }
+    }
+    {
+        struct S([Option<Cell<u32>>; 2]);
+        impl Index<usize> for S {
+            type Output = Option<Cell<u32>>;
+            fn index(&self, idx: usize) -> &Self::Output {
+                &self.0[idx]
+            }
+        }
+
+        const C: S = S([Some(Cell::new(0)), None]);
+        let _ = &C; //~ borrow_interior_mutable_const
+        let _ = &C[0]; //~ borrow_interior_mutable_const
+        let _ = &C.0[0]; //~ borrow_interior_mutable_const
+        let _ = &C.0[1];
+    }
+    {
+        const C: [Option<Cell<u32>>; 2] = [None, None];
+        let _ = &C[0];
+        let _ = &C[1];
+        let _ = &C[2];
+
+        fn f(i: usize) {
+            let _ = &C[i];
+        }
+    }
+    {
+        const C: [Option<Cell<u32>>; 2] = [None, Some(Cell::new(0))];
+        let _ = &C[0];
+        let _ = &C[1]; //~ borrow_interior_mutable_const
+        let _ = &C[2];
+
+        fn f(i: usize) {
+            let _ = &C[i]; //~ borrow_interior_mutable_const
+        }
+    }
+    {
+        let _ = &interior_mutable_const::WRAPPED_PRIVATE_UNFROZEN_VARIANT; //~ borrow_interior_mutable_const
+        let _ = &interior_mutable_const::WRAPPED_PRIVATE_FROZEN_VARIANT;
+    }
+    {
+        type Cell2<T> = Cell<T>;
+        type MyCell = Cell2<u32>;
+        struct S(Option<MyCell>);
+        trait T {
+            type Assoc;
+        }
+        struct S2<T>(T, T, u32);
+        impl T for S {
+            type Assoc = S2<Self>;
+        }
+        type Assoc<X> = <X as T>::Assoc;
+        impl S {
+            const VALUE: Assoc<Self> = S2(Self(None), Self(Some(Cell::new(0))), 0);
+        }
+        let _ = &S::VALUE; //~ borrow_interior_mutable_const
+        let _ = &S::VALUE.0;
+        let _ = &S::VALUE.1; //~ borrow_interior_mutable_const
+        let _ = &S::VALUE.2;
+    }
+}